Thermoforming is a manufacturing process in which a plastic sheet is heated to make it flexible, then formed or shaped with a mold and finally trimmed to create a finished part or product. Vacuum forming and pressure forming are two different types of thermoforming. The main difference between pressure forming and vacuum forming is the number of molds used in the process.
Vacuum forming is the simplest method of thermoforming plastic. It requires a mold and negative pressure to achieve the desired part geometry. It is ideal for parts that only need to be accurately formed on one side, such as shaped food packaging or electronic devices.
There are two main types of molds: the male or positive mold, which is convex, and the female or negative mold, which is concave. For male molds, the plastic sheet is laid over the mold to match the inside dimensions of the plastic part. For female molds, the thermoplastic sheet is placed inside the mold to precisely form the outer dimensions of the part.
- Male mold, defined by convex or positive elements.
- Female mold, defined by concave or negative elements.
In pressure forming, a heated plastic sheet is pressed (hence the name) between two molds instead of being pressed onto a single mold by suction. Pressure forming is ideal for making plastic parts that need to have a very precise shape on both sides and/or require higher pressure (they need to penetrate further or deeper into the mold), such as appliance housings that need to have a nice aesthetic appearance on the outside and high precision on the inside.
How does vacuum forming work?
The successive steps of the vacuum forming process are as follows:
Clamping: A plastic sheet is positioned on an open frame and clamped in place.
Heating: The plastic sheet is softened with a heat source until it reaches the proper forming temperature and becomes pliable.
Vacuuming: The frame holding the heated and pliable plastic sheet is lowered onto a mold and the sheet is pressed against it by a vacuum created on the other side of the mold. Female (or convex) molds must be drilled with tiny holes in the cavities so that the vacuum can effectively shape the thermoplastic sheet into the desired shape.
Cooling: Once the plastic is formed around or inside the mold, it must be cooled. For large parts, fans and/or misting are sometimes used to speed up this stage of the production cycle.
Demolding: When the plastic has cooled, it can be separated from the mold and removed from the frame.
Finishing: The molded part still needs to be cleaned of excess material and its edges may need to be chamfered, sanded or polished.
Vacuum forming is a relatively quick process, with the heating and vacuuming steps usually taking only a few minutes. However, depending on the size and complexity of the parts to be produced, cooling, finishing and mold creation can take considerably longer.
Advantages and disadvantages of vacuum forming
Many manufacturers, designers, and other professionals prefer vacuum forming because it offers flexibility in model shape at a relatively low cost compared to other manufacturing methods. The following is a non-exhaustive list of the advantages of vacuum forming:
Vacuum forming is generally more affordable than other manufacturing methods such as plastic injection molding, especially for small production runs (250 to 300 units per year). The low cost of vacuum forming is due in large part to the lower tooling and prototyping costs it requires. Depending on the surface area of the part to be produced and the dimensions of the clamping frame, tooling for injection molding can cost two to three times more than for the same part made by thermoforming or vacuum forming of plastic.
Vacuum forming allows for a shorter production cycle than other traditional manufacturing methods because the tooling it requires can be made more quickly. The tooling production cycle for vacuum forming is typically twice as short as for injection molding tooling. When 3D printers are used to create the molds, the production time can be even shorter. By improving the efficiency of the production process, vacuum forming allows companies to bring new designs to market faster.
Vacuum forming gives designers and manufacturers the flexibility to test new designs and make prototypes without incurring exorbitant costs or delays. Molds can be made of wood, aluminum, structural foam or 3D printed plastic so they can be replaced and/or modified more easily than with other manufacturing processes.
Vacuum forming also allows designers to offer a wider range of colors and customizations to consumers. This gives many companies the freedom to offer their customers unique designs and make custom products such as dental appliances for an affordable price.
Use of sterile and food-grade materials
Manufacturers often use vacuum forming to produce food-grade containers and parts for the medical industry because the process is compatible with sterilizable or contaminant-resistant plastics. For example, high-density polyethylene (HDPE) is often used in vacuum forming of food containers.
HDPE’s resistance to acidic compounds also makes it a suitable material for vacuum forming of cleaning product containers. Medical grade plastics are used in vacuum forming to create parts that can withstand sterilization processes and meet stringent medical and/or pharmaceutical standards.
Limitations of Vacuum Forming
While vacuum forming has a number of advantages, it also has its limitations. It is only suitable for parts with relatively thin walls and simple geometries. Finished parts may not have a consistent wall thickness, and concave parts with deep recesses are difficult to produce when vacuum forming.
In addition, while vacuum forming is often the most cost-effective choice for small to medium production runs, other plastic manufacturing processes may be cheaper for very large production runs.
The different applications of vacuum forming
Nowadays, it would be hard to go a day without coming into contact with at least one part made by vacuum forming. This manufacturing process is used to produce a wide variety of parts in many different industries. Here are a few examples:
Automotive and transportation
Manufacturers use vacuum forming to create a wide range of parts for cars, buses, boats and airplanes. In cars, everything from bumpers to floor mats and even trunk liners are made by vacuum forming.
Vacuum forming allows car manufacturers to differentiate their production in terms of color or equipment level and gives designers the ability to test new prototypes during development.
Manufacturers also use vacuum forming to produce all kinds of industrial crates and profile packaging for machinery.
Vacuum forming is an ideal manufacturing process for parts that must be placed outdoors or come into contact with a heat source because manufacturers can use a variety of flame retardant or UV resistant materials. For example, polyvinyl chloride (PVC) meeting UL 94 V-0 flammability standards or UV-treated acrylic (PMMA) can both be vacuum formed.
Packaging and Displays
When you buy a product in a form-fitting plastic display, chances are that the display was made by vacuum forming. Razors, toothbrushes, electronics, and makeup or cleaning products are often sold in a vacuum formed package.
Vacuum forming is often used for food packaging because it is compatible with food-grade plastic and can produce parts that are easy to sterilize.
The list of consumer products made by vacuum forming is virtually endless. Manufacturers can create products such as children’s toys, travel items, and home décor using vacuum forming.
Medical parts and components that need to receive antimicrobial treatment or be resistant to contaminants are often manufactured using vacuum forming.
Custom parts and special effects
Vacuum forming is used in a variety of industries to create custom and unique parts such as special effects accessories.